The removal of gaseous contaminants from the flue gases of industrial combustion processes, for example the combustion of fossil fuels in power plants and industrial furnaces, has become increasingly important as the concern over environmental protection has grown.
The "acid" gases NO, NO.sub.2, SO.sub.2, SO.sub.3, HCl, and HF are major contaminants of the atmosphere and are the main causes of acid rain by release of flue gases into the atmosphere. The maximum permissible emission of such gases is regulated by law and must be relatively small.
Apart from the nitrogen compounds, the other gases are generally removed from the flue gas by sorption processes and usually by a subsequent chemical reaction with a basic substance, for example, a calcium compound.
There are many processes for the recovery of sulfur dioxide. Because of the simplicity in handling, dry adsorption appears to be of considerable interest. The dry adsorbents which are most common are active carbon or active coal, which has primacy at the present time, although Ca(OH)2, CaCO.sub.3, CaO and NaHCO.sub.3 are used, the latter compounds also reacting chemically with the adsorbed gas. R. Rasch, "Entwicklungsstand der trockenen Heissreinigung von Rauchgasen", in Maschinenmarkt Wurzburg 85 (1979) 32 pages 620-623 has provided an overview as to this state of the art.
Because the most common of the sorption materials act not only as adsorbents but also as reactants, the process has generally been described as chemisorption. The disadvantage of all of these chemisorption processes is that in the available reaction time for technologically suitable processes, only a relatively thin product layer can be formed on the surface of the sorbent particles so that the core of the latter remains largely unutilized.
This disadvantage can be reduced by the use of particles having a very large specific surface, although it cannot be eliminated.
Investigations have been made into increasing the reaction rate between the gaseous contaminant and the usually alkaline sorbent by increasing the relative moisture content by means of injection of water into the hot flue gas (Noln, P. S., Hendriks, R. V. and Kresovich,, N., "Operation of the LIMB/Humidifier Demonstration Unit at Edgewater (104 MWel)" First Combined FGD and Dry SO.sub.2 Control Symposium, 25 through 28 Oct., 1988, St. Louis, Paper Number 4A-1). Attempts have also been made to increase the residence time by baffling the flue gas stream (Kenakkala, T, "Status of the LIFAC Process" First Combined FGD and Dry SO.sub.2 Control Symposium Oct. 25-28, 1988, St. Louis, Paper Number 4A-3).
A significant improvement of the utilization of the solid sorbents and of the desulfurization degree can be achieved, however, in-accordance with prior art techniques only when the temperature of the flue gas is lowered to a temperature close to the water dewpoint.
There are a number of process proposals for improving the utilization of the adsorbent particles, primarily by repeated use, i.e. recycling the particles which have been used to repeat the process. As a rule, a step of "activation" of the additives is interposed.
Thus in German Open Application DE-OS 26 15 828, it has been proposed to mix the sorbent with the gas stream to be cleaned via venturi duct sections and then to remove the sorbent from the gas stream and reintroduce it into the latter. To improve the sorption action, the gas, before contact with the sorbent has steam added to it. In addition, in this system, the supply of an oxidizing agent has been deemed to be necessary. The raw flue gas should have a temperature of at least 120.degree. C.
In German Open Application DE-OS 26 32 502, a process for improving the adsorption and/or absorption or chemisorption in the removal of contaminants from flue gases has been disclosed in which the adsorbent dust after the separator is collected, is wetted to have a maximum moisture content of 10% and is then returned to the apparatus. The moisture content should preferably lie between 4 and 8% to avoid an encrustation on solid walls of the apparatus. The moistening can be effected by means of cooling air or by cooling the walls of the apparatus.
In German Open Application DE-OS 29 10 537, the regeneration of dry residues from a flue gas cleaning system has been proposed in which the solid residues are comminuted and thereafter directly fed to the reaction system so that the exposed or broken surfaces formed by the comminuting operation make free faces available so that the reaction can take place with the contaminants before the available surfaces are saturated This proposal requires the availability of a comminuting apparatus and process, i.e. additional apparatus which may be expensive, to free up surfaces of the nonreacted core of the solid sorbent.
German Open Application DE-OS 29 34 109 relates to a process in which the solid residues of a combustion process are finely milled and thereafter blown into the reaction stage along a flue gas path. Here as well a mechanical comminution is required to make the surface of the solid as large as possible.
German Open Application DE-OS 31 13 788 Al effects a milling of the incompletely utilized sorbent withdrawn from a dry desulfurization process and supplies them to a wet desulfurization apparatus so as to obtain the highest possible efficiency of the sorption process. The utilization of the sorbent can be increased with simultaneous increasing of the degree of desulfurization.
The German Open Application DE-OS 31 23 064 proposes an improvement in the utilization of chemisorbents in that, following a passage of the chemisorption agent through the apparatus and before recycling to the apparatus, the chemisorbent is processed in a steam jet mill.
In the steam jet mill not only is there a comminution to new surfaces but also these new surfaces are found to be activated by moisture.
Austrian patent 385 210 describes a process in which an absorption agent utilized once is recovered from a dry flue gas desulfurization process together with the ash from the fuel and is activated for use in a second path with improved utilization. The activation of this "secondary" adsorbent can be effected with water and/or steam. A mechanical milling improves the activation further. The so activated solids are fed to the flue gas to be desulfurized at a temperature of 100.degree. to 700.degree. C.
U.S. Pat. No. 4,724,130 describes the addition of a promoter to the solid sorbent so as to improve the effectiveness of the latter. By scrubbing the recovered solid, the promoter, which is expensive or can be detrimental to the environment when deposited on waste dumps can be recovered with the scrubbing water and can be regenerated. The sorbent itself is in part recovered, dried and recycled.
In Austrian Patent 380,406, a process is described in which the lime additive which is employed for the desulfurization, is injected into the combustion chamber and recovered from the flue gases, comminuted and used again to treat the flue gas after treatment, in turn, with dry steam. There is also a series of proposals for the regeneration and/or activation of solid sorbents which have been used once, whether these are obtained from the lime additive processes (Austrian Application A 2935/83 and Austrian Patent 385 210) in the combustion chamber of a steam generator or are obtained from the already cooled flue gas stream. All of these process have been found to have some disadvantage. Either a mill or mechanical treatment is required or the sorbent to be regenerated must be moist. Further, one process may require a flue gas temperature in excess of 100.degree. C. while another may require the cooling of the flue gas to a temperature (T.sub.p +5.degree. C.)&lt;T&lt;(T.sub.p +50.degree. C.), in which T is the flue gas temperature and T.sub.p the dewpoint of water in the flue gas.